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This module performs conversions between Python values and C structs represented
as Python bytes objects. This can be used in handling binary data
stored in files or from network connections, among other sources. It uses
Format Strings as compact descriptions of the layout of the C
structs and the intended conversion to/from Python values.

Note

By default, the result of packing a given C struct includes pad bytes in
order to maintain proper alignment for the C types involved; similarly,
alignment is taken into account when unpacking. This behavior is chosen so
that the bytes of a packed struct correspond exactly to the layout in memory
of the corresponding C struct. To handle platform-independent data formats
or omit implicit pad bytes, use standard size and alignment instead of
native size and alignment: see Byte Order, Size, and Alignment for details.

Several struct functions (and methods of Struct) take a buffer
argument. This refers to objects that implement the Buffer Protocol and
provide either a readable or read-writable buffer. The most common types used
for that purpose are bytes and bytearray, but many other types
that can be viewed as an array of bytes implement the buffer protocol, so that
they can be read/filled without additional copying from a bytes object.

Unpack from the buffer buffer (presumably packed by pack(fmt,...))
according to the format string fmt. The result is a tuple even if it
contains exactly one item. The buffer’s size in bytes must match the
size required by the format, as reflected by calcsize().

Unpack from buffer starting at position offset, according to the format
string fmt. The result is a tuple even if it contains exactly one
item. The buffer’s size in bytes, minus offset, must be at least
the size required by the format, as reflected by calcsize().

Iteratively unpack from the buffer buffer according to the format
string fmt. This function returns an iterator which will read
equally-sized chunks from the buffer until all its contents have been
consumed. The buffer’s size in bytes must be a multiple of the size
required by the format, as reflected by calcsize().

Format strings are the mechanism used to specify the expected layout when
packing and unpacking data. They are built up from Format Characters,
which specify the type of data being packed/unpacked. In addition, there are
special characters for controlling the Byte Order, Size, and Alignment.

By default, C types are represented in the machine’s native format and byte
order, and properly aligned by skipping pad bytes if necessary (according to the
rules used by the C compiler).

Alternatively, the first character of the format string can be used to indicate
the byte order, size and alignment of the packed data, according to the
following table:

Character

Byte order

Size

Alignment

@

native

native

native

=

native

standard

none

<

little-endian

standard

none

>

big-endian

standard

none

!

network (= big-endian)

standard

none

If the first character is not one of these, '@' is assumed.

Native byte order is big-endian or little-endian, depending on the host
system. For example, Intel x86 and AMD64 (x86-64) are little-endian;
Motorola 68000 and PowerPC G5 are big-endian; ARM and Intel Itanium feature
switchable endianness (bi-endian). Use sys.byteorder to check the
endianness of your system.

Native size and alignment are determined using the C compiler’s
sizeof expression. This is always combined with native byte order.

Standard size depends only on the format character; see the table in
the Format Characters section.

Note the difference between '@' and '=': both use native byte order, but
the size and alignment of the latter is standardized.

The form '!' is available for those poor souls who claim they can’t remember
whether network byte order is big-endian or little-endian.

There is no way to indicate non-native byte order (force byte-swapping); use the
appropriate choice of '<' or '>'.

Notes:

Padding is only automatically added between successive structure members.
No padding is added at the beginning or the end of the encoded struct.

No padding is added when using non-native size and alignment, e.g.
with ‘<’, ‘>’, ‘=’, and ‘!’.

To align the end of a structure to the alignment requirement of a
particular type, end the format with the code for that type with a repeat
count of zero. See Examples.

Format characters have the following meaning; the conversion between C and
Python values should be obvious given their types. The ‘Standard size’ column
refers to the size of the packed value in bytes when using standard size; that
is, when the format string starts with one of '<', '>', '!' or
'='. When using native size, the size of the packed value is
platform-dependent.

Format

C Type

Python type

Standard size

Notes

x

pad byte

no value

c

char

bytes of length 1

1

b

signedchar

integer

1

(1),(3)

B

unsignedchar

integer

1

(3)

?

_Bool

bool

1

(1)

h

short

integer

2

(3)

H

unsignedshort

integer

2

(3)

i

int

integer

4

(3)

I

unsignedint

integer

4

(3)

l

long

integer

4

(3)

L

unsignedlong

integer

4

(3)

q

longlong

integer

8

(2), (3)

Q

unsignedlonglong

integer

8

(2), (3)

n

ssize_t

integer

(4)

N

size_t

integer

(4)

f

float

float

4

(5)

d

double

float

8

(5)

s

char[]

bytes

p

char[]

bytes

P

void*

integer

(6)

Changed in version 3.3: Added support for the 'n' and 'N' formats.

Notes:

The '?' conversion code corresponds to the _Bool type defined by
C99. If this type is not available, it is simulated using a char. In
standard mode, it is always represented by one byte.

The 'q' and 'Q' conversion codes are available in native mode only if
the platform C compiler supports C longlong, or, on Windows,
__int64. They are always available in standard modes.

When attempting to pack a non-integer using any of the integer conversion
codes, if the non-integer has a __index__() method then that method is
called to convert the argument to an integer before packing.

Changed in version 3.2: Use of the __index__() method for non-integers is new in 3.2.

The 'n' and 'N' conversion codes are only available for the native
size (selected as the default or with the '@' byte order character).
For the standard size, you can use whichever of the other integer formats
fits your application.

For the 'f' and 'd' conversion codes, the packed representation uses
the IEEE 754 binary32 (for 'f') or binary64 (for 'd') format,
regardless of the floating-point format used by the platform.

The 'P' format character is only available for the native byte ordering
(selected as the default or with the '@' byte order character). The byte
order character '=' chooses to use little- or big-endian ordering based
on the host system. The struct module does not interpret this as native
ordering, so the 'P' format is not available.

A format character may be preceded by an integral repeat count. For example,
the format string '4h' means exactly the same as 'hhhh'.

Whitespace characters between formats are ignored; a count and its format must
not contain whitespace though.

For the 's' format character, the count is interpreted as the length of the
bytes, not a repeat count like for the other format characters; for example,
'10s' means a single 10-byte string, while '10c' means 10 characters.
If a count is not given, it defaults to 1. For packing, the string is
truncated or padded with null bytes as appropriate to make it fit. For
unpacking, the resulting bytes object always has exactly the specified number
of bytes. As a special case, '0s' means a single, empty string (while
'0c' means 0 characters).

When packing a value x using one of the integer formats ('b',
'B', 'h', 'H', 'i', 'I', 'l', 'L',
'q', 'Q'), if x is outside the valid range for that format
then struct.error is raised.

The 'p' format character encodes a “Pascal string”, meaning a short
variable-length string stored in a fixed number of bytes, given by the count.
The first byte stored is the length of the string, or 255, whichever is
smaller. The bytes of the string follow. If the string passed in to
pack() is too long (longer than the count minus 1), only the leading
count-1 bytes of the string are stored. If the string is shorter than
count-1, it is padded with null bytes so that exactly count bytes in all
are used. Note that for unpack(), the 'p' format character consumes
count bytes, but that the string returned can never contain more than 255
bytes.

For the '?' format character, the return value is either True or
False. When packing, the truth value of the argument object is used.
Either 0 or 1 in the native or standard bool representation will be packed, and
any non-zero value will be True when unpacking.

Return a new Struct object which writes and reads binary data according to
the format string format. Creating a Struct object once and calling its
methods is more efficient than calling the struct functions with the
same format since the format string only needs to be compiled once.